Studies of the regulation of human cytotoxic T lymphocyte (CTL) development were continued. a) The high efficiency cloning system for human CTL precursors developed last year was employed for limiting dilution analysis of CTL differentiation. Using this technique and others, we furthered our studies regarding IL-4 suppression of CTL development. Four principal findings developed. First, the suppressive effect of IL-4 was found to be exerted directly on the CD8 plus CTL precursor. Second, the partial suppression due to IL-4 was found to be due to complete blockage of CTL development in a subset of CTL precursors rather than partial block in all precursors. The nature of the subset is under study. Third, suppression occurred if and only if IL-4 was present during activity, not earlier or later. Fourth, presence of IL-4 during activation of CD8 plus lymphocytes resulted in an increase in the frequency of proliferating, noncytotoxic CD8 plus cells. These cells are now under study. b) Human CTL lines have been developed and grown in the presence of IL-4 rather than IL-2. Their growth and differentiation have been shown to be IL-2 independent. The specificity and range of their cytotoxicity differs from lines grown in IL-2. c) IL-12 has been found to have significant effects upon the growth and differentiation of human CTL. These effects are under active investigation. d) RT-PCR techniques for analyzing key molecular events in CTL development are under development.